1. Field of the Invention
The invention relates to a method for the conversion of renewable fats and oils (triacylglycerides, or TAGs) to hydrocarbons. The invention also accommodates the production of renewable fuels from either fatty acid methyl ester (FAME) or free fatty acids (FFAs). The fatty acids, for instance, may be derived from TAGs via hydrolysis by a number of methods, including steam hydrolysis. The oils may be derived from plants, animals, or algae, or mixtures thereof. The method is applicable to the manufacture of liquid transportation fuels, for example, especially gasoline, naphtha, kerosene, jet, and diesel fuels. The method is also applicable to the manufacture of other hydrocarbons.
2. Background of the Invention
Increasing costs for petroleum-derived fuels are driving interest in alternative feedstocks. Additionally, concern over increasing atmospheric carbon dioxide levels has spawned interest in “carbon-neutral” fuels. One possible solution to both of these issues is the utilization of TAG feedstocks for the production of hydrocarbon-based transportation fuels.
Certain TAGs are already utilized as feedstocks for the production of “biodiesel.” In this process, the TAG is transesterified with methanol to provide a FAME and glycerine. The FAME is separated, purified, and sold as an additive, supplementing petroleum-derived diesel fuel. FAME diesel additives provide certain specific benefits to their use (i.e., lubricity), but suffer serious physical limitations when used as the sole fuel and not as a blendstock (i.e., cold-flow properties).
FAME diesel fuel represents a first-generation bio-derived fuel. The shortcomings of this generation of fuel are directly related to the fuel-possessing oxygen functionality. A second-generation fuel possesses no oxygen functionality, providing a more petroleumlike product with respect to elemental composition, and is oftentimes termed “renewable diesel.”
Recent publications and patents have described the conversion of TAG to hydrocarbon fuels via technology oftentimes referred to as “hydrodeoxygenation.” This technology converts the fatty acid portion of a TAG to a normal hydrocarbon either of a carbon number equal to the original fatty acid or to a hydrocarbon possessing one carbon less than the original fatty acid. The glycerine portion of the TAG is most often converted to propane or otherwise lost within the process.
The glycerine portion of the TAG possesses economic value in itself greater than that of propane and, as such, could be an important economic by-product from an overall process that would provide glycerine as a by-product.
Certain patents list strategies for limiting the acidity of the fuel that is produced. This can include recycle of the product with fresh feedstock over the catalyst bed and limiting the total acidity of the product introduced to the catalyst.
A major difference between fatty acid and TAG is the nature of the acid functionality present in each compound. For the TAG, the acid is present as an ester functionality. For the fatty acid, the acid is present as a carboxylic acid. It is well established that ester functionality is more easily reduced to saturated hydrocarbon via hydrogenation technology than is the carboxylic acid functionality. This limits the amount of fatty acid that may be present in the feedstock and feedstock blends.
One method describes the conversion of depitched tall oil to a diesel fuel additive (see generally Canadian Patent 2,149,685). The method describes a hydrodeoxygenation process utilizing a hydrotreating catalyst. The catalyst is prepared by presulfiding. The sulfided nature of the catalyst may be maintained by adding sulfur to the tall oil feedstock at a level of 1000 ppm. The doping agent is carbon disulfide. The hydrodeoxygenation conversion is then performed at 410° C. and 1200 psi.
Another method describes the preparation of a diesel fuel from a vegetable TAG oil (see generally U.S. Patent Application 2007/0010682). The TAG oil is doped with 50 to 20,000 ppm sulfur. The hydrodeoxygenation step is performed between 580 and 725 psi and 305° and 360° C.
Accordingly, there is a need for a method of producing paraffinic hydrocarbons from a feedstock comprising TAGs without the need for presulfiding the hydrotreating catalyst or doping the feedstock with sulfur. There is a need for a hydrotreating process where the resulting hydrocarbon chain lengths are distributed similarly to those in conventional petroleum-derived fuels. Additionally, there is a need for a method that is tailored to the efficient reduction of fatty acid to hydrocarbon, with no limitation to the amount of fatty acid that may be present in the feedstock blend.